J. Phys. Chem. A 2007, 111, 13387-13396
13387
Probing the Dependence of Long-Range, Four-Atom Interactions on Intermolecular Orientation. 1. Molecular Hydrogen and Iodine Monochloride Joshua P. Darr,† Andrew C. Crowther,‡ and Richard A. Loomis*,§ Department of Chemistry, Washington UniVersity, One Brookings DriVe, CB 1134, Saint Louis, Missouri 63130
Sara E. Ray and Anne B. McCoy§ Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed: August 11, 2007; In Final Form: October 2, 2007
The dependence of the long-range interactions between molecular hydrogen and iodine monochloride on the geometry between the molecules is investigated. Laser-induced fluorescence and action spectroscopy experiments have identified multiple conformers of the o,p-H2‚‚‚I35,37Cl(X,V′′)0) van der Waals complexes. A conformer with the hydrogen molecule localized at the iodine end of the dihalogen, most likely with C2V symmetry, is significantly more stable than an asymmetric conformer with the hydrogen localized in the well oriented orthogonally to the I-Cl bond axis, D0′′ ) 186.4(3) cm-1 versus 82.8(3) e D0′′ e 89.6(3) cm-1. Complexes containing the o-H2(j)1) species are more strongly bound than those with p-H2(j)0). The electronically excited o,p-H2‚‚‚I35Cl(A,V′) and o,p-H2‚‚‚I35,37Cl(B,V′) complexes are found to have preferred asymmetric structures with binding energies bracketed between 73.7-80.5 and 69.5-76.3 cm-1 for o-H2‚‚‚ I35Cl(A,V′)23) and o-H2‚‚‚I35Cl(B,V′)3), respectively. Calculations of the H2‚‚‚I35Cl(B,V′)3) intermolecular vibrational energies and probability amplitudes undertaken using a scaled He + ICl(B,V′)3) potential enable us to make tentative assignments of the excited-state levels experimentally accessed.
1. Introduction This paper expands on our previous efforts that have focused on characterizing the multidimensional potential energy surfaces (PESs) of ground and excited-state three-atom, rare gasdihalogen, Rg‚‚‚XY, van der Waals complexes.1-6 Here, we present spectroscopic results for the four-atom H2‚‚‚ICl complex that reveal information concerning the nature of the dominant long-range interactions and the dependence of these interactions on intermolecular orientation. Four-atom systems present both new opportunities and challenges. For instance, the presence of two diatomic subunits increases the degrees of freedom that must be considered, especially with respect to internal rotational or bending motion. At the same time, having two diatomic subunits also allows us to more thoroughly study the role of electrostatic interactions in this class of van der Waals complexes. Rovibronic spectra of ortho(o)-H2/D2‚‚‚I2 and para(p)-H2/ D2‚‚‚I2 complexes recorded in the I2 B-X region were reported by Kenny et al.7 In those spectra, they observed two features shifted by ≈15-20 cm-1 to higher energy than the corresponding I2 B-X, V′-0 monomer feature; one feature was attributed to transitions of an o-H2/D2‚‚‚I2 complex and the other to transitions of a p-H2/D2‚‚‚I2 complex. The intensity of the o-H2‚‚‚I2 feature was approximately 3 times more intense than the p-H2‚‚‚I2 feature, in accord with the 3:1 relative population of o-H2 to p-H2 expected for a room-temperature sample of † Present address: JILA, University of Colorado, Campus Box 440, Boulder, CO 80309, USA. ‡ Present address: Department of Chemistry, University of Wisconsins Madison, 1101 University Ave., Madison, WI 53706, USA. § Electronicmail: R.A.L.,
[email protected];A.B.M.,
[email protected].
hydrogen. An additional feature, shifted by ≈75 cm-1 from the monomer feature, was observed and attributed to transitions of the o-H2/D2‚‚‚I2 complex to an excited intermolecular vibrational level. On the basis of similarities of the spectral shifts and rotational contours of the H2‚‚‚I2 features with those of previously characterized Rg‚‚‚I2 systems, the average geometries of the ground-state o,p-H2/D2‚‚‚I2(X,V′′)0) complexes were presumed to be similar to the T-shaped Rg‚‚‚XY structure with the H2 or D2 molecules localized in the toroidal well positioned orthogonally to the I-I bond axis. The lowest intermolecular level within each o,p-H2/D2 + I2(B,V′) excited-state PES was also believed to have a similar geometry. The binding energies for the ground-state o-H2‚‚‚I2(X,V′′)0) and excited-state o-H2‚‚‚I2(B,V′)28) complexes were measured to be 92.3 and 74.4 cm-1, respectively. If the H2‚‚‚I2(X,V′′)0) complexes have on average C2V symmetry, such as
then quadrupole-quadrupole forces would be the dominant electrostatic interaction. In contrast, dipole-quadrupole and dipole-induced dipole dispersion forces will most likely be the dominant electrostatic interactions that dictate the properties of the multidimensional PESs associated with heteronuclear H2‚‚‚XY complexes, such as H2‚‚‚ICl. The somewhat classical use of electrostatic forces in explaining the observed intermolecular interactions is admittedly overly simplistic because the electronic orbitals of the molecules and the partial exchange of charge via donor-acceptor interactions play key roles in dictating preferred geometries of these complexes. However, this picture gives a good first-order approximation of the
10.1021/jp076465u CCC: $37.00 © 2007 American Chemical Society Published on Web 12/04/2007
13388 J. Phys. Chem. A, Vol. 111, No. 51, 2007 interactions involved. Consequently, a comparison of the preferred geometries and intermolecular vibrational energies for the H2‚‚‚I2 and H2‚‚‚ICl complexes should provide insight into the contribution of electrostatic forces in the long-range, intermolecular interactions for these four-atom systems. We present here the results from laser-induced fluorescence (LIF) and action spectroscopy experiments aimed at characterizing the ground- and excited-state interactions of H2 with ICl. Discrete rovibronic features associated with transitions of two different ground-state H2‚‚‚ICl(X,V′′)0) conformers are observed. One conformer most likely has a C2V, near-prolate symmetric top geometry with the H2 moiety localized at the iodine end of the dihalogen, similar to the structures of the linear Rg‚‚‚XY(X,V′′)0) conformers.2-4,6,8 The other conformer is most likely asymmetric with the H2 molecule localized in the toroidal potential well oriented orthogonally to the I-Cl bond axis, similar to the structures of the near T-shaped Rg‚‚‚ XY(X,V′′)0) conformers.2-4,6,8 By performing experiments with a carrier gas consisting of predominantly p-H2 in He, we have also identified which H2‚‚‚ICl features can be associated with transitions of complexes containing either o-H2 or p-H2. The binding energy of each ground-state conformer is determined, revealing that the C2V conformer is much more strongly bound than the asymmetric conformer for both the o-H2‚‚‚ICl(X,V′′)0) and p-H2‚‚‚ICl(X,V′′)0) complexes. The excited-state o,p-H2‚‚‚ICl(B,V′) and o,p-H2‚‚‚ICl(A,V′) complexes are both found to have an average asymmetric geometry in the most strongly bound intermolecular vibrational level with no indication of other minima along the angular coordinate about the dihalogen. These observations are consistent with earlier studies of a variety of Rg‚‚‚XY(B,V′) complexes.2-4,6,8 To assign the numerous excited-state intermolecular vibrational levels associated with the observed transitions, the energies and probability amplitudes of the bound H2‚‚‚ICl(B,V′)3) levels were calculated. Last, comparisons of the energetics and geometries of the o,p-H2‚‚‚ICl complex with other H2‚‚‚diatom systems, including H2‚‚‚HF,9 H2‚‚‚HCl,10 H2‚‚‚OH,11 H2‚‚‚NH,12 and H2‚‚‚CN,13-15 help to elucidate the role of electrostatic interactions in these four-atom systems. 2. Experimental Section Ground-state H2‚‚‚ICl(X,V′′)0) van der Waals complexes were stabilized and laser-induced fluorescence (LIF) and action spectra of the complexes were recorded in a manner similar to that described previously for the He‚‚‚ICl and Ne‚‚‚ICl complexes.1,3,16 As with Ne‚‚‚ICl, the carrier gas of interest was diluted in He so that the propensity for forming higher order complexes, in this case (H2)m‚‚‚ICl(X,V′′)0) with m g 2, was minimized. Numerous LIF spectra were acquired with systematically varied expansion conditions to optimize the signal-tonoise levels of the H2‚‚‚ICl features and to aid in determining spectroscopic assignments of the features. Specifically, by varying the reduced distances downstream at which the spectra were recorded, Z ) x/d, where x is the downstream distance and d is the nozzle diameter, 0.89(3) mm, we characterized the dependence of the intensities of each H2‚‚‚ ICl feature on the local temperature within the expansion. The local rotational temperature associated with the conditions utilized when acquiring the spectra was determined by recording an LIF spectrum of the I35Cl B-X, 2-0 monomer band and fitting the rotational contour assuming a Boltzmann-rotational distribution. The H2/He backing pressure and the concentration of H2 in He were also varied to determine the effects of varying the H2/ICl and H2/H2‚‚‚ICl collision frequency on the formation
Darr et al. of different ground-state conformers. The optimum conditions for recording LIF spectra of the H2‚‚‚ICl complexes in both the ICl B-X and A-X regions were obtained when a carriergas mixture of 5% H2 in He was used with a 7.9 bar backing pressure and the laser crossing the expansion at Z ≈ 11. Unless explicitly stated otherwise, these conditions were used in acquiring the presented spectra. The I35Cl(X,V′′)0) rotational temperature determined when these conditions were used was typically ≈1.8 K. High-purity p-H2(j)0) in He was used as a carrier gas in some experiments to assist in distinguishing the p-H2‚‚‚ICl features from the o-H2‚‚‚ICl features. The p-H2 was prepared in the McCall laboratory in the Chemistry Department at the University of Illinois, Urbana-Champaign, by cooling normal(n)-H2(j)even,odd) in liquid He in the presence of a metal catalyst. The resulting p-H2(j)0) was collected and then diluted with He to an ≈5% concentration. A small fraction of the p-H2,
